This invention relates generally to an electrophotographic copying machine, and more particularly to a corona generating apparatus useful for charging a photoconductive surface to a substantially uniform potential, for electrostatically transferring a toner image from the photoconductive surface to a transfer sheet, or for neutralizing the charge on the photoconductive conductive surface.
Conventional corona generators used heretofore in electrophotographic copying machines include the corotron type schematically shown in a sectional view of FIG. 1(a) and the scorotron type shown in FIG. 1(b). In each of the generators, a high a-c or d-c voltage is applied to one or more corona discharge electrodes 1 composed of metallic thin wires such as tungsten to generate corona ions around the peripheries thereof, and the ions are emitted toward the open side of a shield 2 which is composed of a metallic plate such as aluminum and surrounds the corona discharge electrodes 1 so as to shield the electric field of the discharge electrodes, thereby creating a corona current flow from the electrodes to a nonconductive member spaced therefrom for charging or discharging. Although the corotron type is suited to offer a high ion emission density, it is deficient in attaining uniformity and stability in ion emission density. And also, in the scorotron type, there is difficulty in ensuring a high emission density though its uniformity is improved by the provision of a grid 3 on the open side of the shield electrode 2.
In the scorotron type as well as in the corotron type, the performance achievable is not satisfactory in charging and discharging a photosensitive medium of, for example, an electrophotographic copying machine where high uniformity and stability are important requisites with respect to the ion emission density. And particularly a great problem exists in that the uniformity is reduced with the lapse of operation time.